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The properties of underluminous Type Ia supernovae (SNe Ia) of the 91bg subclass have yet to be theoretically understood. Here, we take a closer look at the structure of the dim SN Ia 2005bl. We infer the abundance and density profiles needed to reproduce the observed spectral evolution between −6 d and  +12.9 d  with respect to  B  maximum. Initially, we assume the density structure of the standard explosion model W7; then we test whether better fits to the observed spectra can be obtained using modified density profiles with different total masses and kinetic energies. Compared to normal SNe Ia, we find a lack of burning products especially in the rapidly expanding outer layers  ( v ≳ 15 000 km s −1 ) . The zone between ∼8500 and 15 000 km s −1  is dominated by oxygen and includes some amount of intermediate‐mass elements. At lower velocities, intermediate‐mass elements dominate. This holds down to the lowest zones investigated in this work. This fact, together with negligible‐to‐moderate abundances of Fe‐group elements, indicates large‐scale incomplete Si burning or explosive O burning, possibly in a detonation at low densities. Consistently with the reduced nucleosynthesis, we find hints of a kinetic energy lower than that of a canonical SN Ia: the spectra strongly favour reduced densities at  ≳13 000 km s −1   compared to W7, and are very well fitted using a rescaled W7 model with original mass  (1.38 M ⊙ ) , but a kinetic energy reduced by ∼30 per cent (i.e. from  1.33 × 10 51   to  0.93 × 10 51  erg ).

Spectral analysis of the 91bg‐like Type Ia SN 2005bl: low luminosity, low velocities, incomplete burning